This directive is used by some automatic code generation tools which produce C source files from a file written in another language. In that case, #line directives may be inserted in the generated C file referencing line numbers and the file name of the original (human-editable) source file.

The line number following the directive #line __LINE__ is implementation-defined (there are two possible values that __LINE__ can expand to in this case: number of endlines seen so far, or number of endlines seen so far plus the endline that ends the #line directive)